The present invention relates to an electromagnet having a magnetic shield for use in a nuclear magnetic resonance diagnostic device which requires a highly uniform magnetic field.
In a conventional electromagnet for use in a nuclear magnetic resonance diagnostic device, it is necessary to generate a uniform and intense magnetic field in an imaging space in an opening of the electromagnet. In generating such magnetic field, there is a considerable problem that flux leakage adversely affects peripheral devices. In order to solve this problem, it has been usual to enclose the electromagnet by a ferromagnetic material to shield it externally.
FIG. 4 shows a conventional electromagnet exemplified by one shown in Japanese Patent Laid-open No. 1437/1983, in which a field winding 1 is wound on a bobbin 2 and shielded by a magnetic shield 3. The magnetic shield 3 is composed of a cylindrical yoke 4 of ferromagnetic material having opposite ends closed by end plates 5 having central openings 6, respectively. A circular disc spacer 7 is disposed between the field coil 1 and each end plate 5.
With such conventional electromagnet, it is possible to reduce a flux leakage effectively. Since the electromagnet for use in the nuclear magnetic resonance diagnostic device requires a highly uniform magnetostatic field, the field coil 1 takes the form of two or more divided coils configulated and arranged by taking the magnetic shield 3 into consideration such that the required uniform magnetic field is obtained. The field coil 1 and the magnetic shield 3 are arranged coaxially and symmetrically in an axial direction. Otherwise, a magnetic gradient would be produced to degrade the uniformity of magnetic field.
In a case of a super conductive electromagnet, for example, it is necessary to form at least one opening in the shield, through which a coolant such as liquid helium is supplied to maintain the field coil 1 at very low temperature and/or for electric conductors to pass to excite the field coil.
FIG. 5a shows another conventional electromagnet having a magnetic shield 3 formed in an end plate with a side hole 10 through which a port portion 9 of a low temperature container 8 is accessed, and FIG. 5b shows another conventional electromagnet having a magnetic shield 3 formed in a yoke portion 4 thereof with such hole 10. Other portions of this example are the same as those used in the device shown in FIG. 4.
In the conventional electromagnet having the magnetic shield as mentioned above, the side hole which is not symmetrical causes a magnetic gradient to be produced, resulting in a degradation of uniformity of the magnetic field produced thereby. In order to compensate this degradation of magnetic field uniformity, the output of a correction coil is increased necessarily. Further, in order to restrict the magnetic field gradient while keeping the symmetrical structure of the magnet, it is necessary to form another side hole 10a in a symmetrical position to the side hole 10 as shown in FIG. 6. However, the additional side hole may cause the shielding effect of the magnetic shield to be lowered necessarily.